powerpc/pseries/lparcfg: convert to papr_sysparm API

/proc/powerpc/lparcfg derives the LPAR name and SPLPAR characteristics
it reports using bare calls to the RTAS ibm,get-system-parameter
function. Convert these to the higher-level papr_sysparm API, which
handles the tedious details.

While the SPLPAR string parsing code could stand to be updated, that
should be done in a separate change. It is minimally modified here to
reduce the risk of changing behavior.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-16-26929c8cce78@linux.ibm.com